Why don't enchanter shields have a burst duration?

Honestly, most shields in the game last roughly 3-5 seconds and account for a massive percent of an assassin's and burst mage's damage. While it's not inherently bad, and can be a way to counter their burst, they last way longer than needed.

I feel like support shields should take some skill and have thought to them. Being able to give a squishy target an extra 1/3 of their HP in shields for the duration of an entire fight isn't healthy, and turns what should be a downside of having high damage into something covered by point and click skills.

I feel like supports should give shields, but last just long enough to not get burst into oblivion by an assassin or burst mage combo when played right. They shouldn't be a set it and forget it ability.